1. Field of the Invention
The present invention relates generally to respirometers and more particularly to an on-line respirometer for quickly and accurately evaluating the biological activity levels for suspended biological matter.
2. Description of the Prior Art
Methods of sewage treatment typically involve the retention of the treated sewage in a processing tank until the biological population (bacteria) within the sewage has consumed a sufficient amount of the available nutrients so that the biological population will not grow to an unacceptable level when the sewage is released. Treated sewage is typically released into fresh water disposal areas. If the sewage is introduced to the disposal area containing an excessive biological population, the biological population will continue to grow rapidly and consume the oxygen in the body of water at a rate higher than that in which oxygen is absorbed by the body of water. Eventually, the dissolved oxygen within the body of water will be depleted, killing any animals living within the body of water. Therefore, it is crucial that the rate of oxygen consumption in the sewage be known prior to its disposal.
Respirometers have been commonly used to monitor the oxygen consumption rate of treated sewage prior to its disposal. Many different respirometer designs and methods of using the same have been incorporated in the treatment of sewage. Most prior art respirometers usually comprise a reaction vessel, a method of inserting a sewage slurry sample into the vessel, and a means for monitoring the pressure change in the reaction vessel.
One such type of respirometer requires the user to insert the slurry sample (containing a measured amount of a biological population and sewage). The user then introduces a measured amount of oxygen to the reaction vessel and aerates the slurry sample. The pressure drop is then measured, and the entire process is repeated a number of times to create a long-term record of oxygen uptake. This type of respirometer suffers from a number of problems. First, this method of respirometry requires the sewage to be diluted with aerated water prior to measurements being taken. Second, the user is required to test sequential batch reactions and is unable to create a true record of continuous oxygen uptake from the sample.
In another type of on-line respirometry, the biological population is first mixed with oxygen-enriched air so that the dissolved oxygen concentration remains relatively constant throughout the test. When the sewage sample is added, the dissolved oxygen concentration will begin to decrease as the biological population consumes the oxygen within the slurry. However, the level of oxygen concentration will typically begin to increase as the rate of oxygen transferred into the sample exceeds the rate of oxygen uptake by the biological population. This process is repeated several times to generate a consumption curve. One problem with this method of respirometry is the large number of repetitive measurements required to obtain a meaningful indication of the rate of oxygen uptake. Another problem is that the results are dependent on the mass transfer characteristics, which are subject to considerable variability over time.
In another common method of respirometry, the oxygen uptake is measured in a continuous fluid flow system, which can either be mobile or fixed with respect to the biological population flow. The decreases in dissolved oxygen concentration are measured and recorded on a continuous basis. The biological oxygen demand is derived by correlating the oxygen uptake rate to a standard measure of biochemical oxygen demand. This method of respirometry is problematic in that the decrease in oxygen concentration across the reaction vessel must be relatively large in order to obtain an accurate measurement of the oxygen uptake. This lack of sensitivity may result in inaccurate test results.
In yet another method of respirometry, both the fluid and gas phases of the test medium are tested in continuous flow streams. The consumption of oxygen is determined by the volume of fresh air or oxygen flowing into the vessel and the dissolved oxygen level in either the headspace or the slurry sample. One of the problems with such a system is that the change in oxygen content of the air stream must be relatively large to provide an accurate calculation of oxygen consumption by the biological population. Again, this lack of sensitivity may skew the final test results.
Each of the aforementioned methods of respirometry require large changes in dissolved oxygen concentration or headspace oxygen content to produce measurements of oxygen uptake. However, U.S. Pat. No. 6,063,617 discloses a method of respirometry that does not require the measurement of such large changes in order to determine the rate of oxygen uptake for a given sample. That method involves the measurement of the amount of supplementary oxygen that must be injected into a reaction vessel to maintain a consistent dissolved oxygen concentration in the slurry sample. The oxygen is automatically injected into the vessel in response to the changes of oxygen that occur within the sample. However, this method of respirometry is also susceptible to a lack of sensitivity and high levels of complexity due to multiple, sequential instrument error and requires long periods of time to measure respiration changes.
Accordingly, what is needed is an on-line respirometer and method of using the same that simply and accurately measures the rate of oxygen uptake of a biological population in a slurry sample within a time frame which will allow its use to assist in the control of variables which can accompany biological needs change during events such as diurnal flow and composition variations.